The present invention relates to image processing and more particularly to a method for processing gray scale pel values to facilitate subsequent data compression.
Because travel costs are rising and because a traveler's time in transit can seldom be used productively, there is an increasing interest in the use of teleconferencing as an alternative to face to face business meetings between people from different locations. In a typical teleconferencing system, people in different cities or even different countries meet in special teleconferencing rooms at their respective home locations. Each room normally includes a room camera for capturing a wide angle view of the people, a document camera which can be focused on letters, drawings or other documents, a room monitor for permitting people in one room to see those in the other, and a document monitor for viewing documents being presented in the other room. Communications between the two rooms are established over conventional teleprocessing links, such as leased or switched telephone lines or satellite communication channels.
To reduce communication costs, freeze-frame teleconferencing techniques may be employed. The video image captured by a room camera is updated only periodically, either at fixed intervals or on command of an operator. People at the receiver see the same "frozen" room image between updates. Audio signals are transmitted on a real time basis so that there is no perceptible delay in voice communications. Document images are updated only when the person presenting a document pushes a "send" button in the teleconferencing room.
After a "send" button is pushed, the image of the presented document does not appear immediately on the display or monitor in the receiving teleconferencing room. A finite period of time is required to scan, capture and process image data at the originating teleconferencing room, to transmit the processed data over teleprocessing links and to process data at the receiving teleconferencing room in order to reconstruct the image of the presented document. The length of the delay can be critical in a teleconferencing system. Delays exceeding a few seconds produce unnatural pauses in the smooth flow of a business meeting.
The length of the delay is directly related to the amount of data which must be transmitted in order to construct an acceptable video image at the receiving teleconferencing room and is inversely related to the bandwidth of the teleprocessing link over which the data must be transmitted. Delay can be reduced by using a higher bandwidth channel. However, communication costs are a direct function of required bandwidth making it desirable to utilize low bandwidth links; e.g., conventional telephone lines.
Delay time and communication costs can be reduced by compressing the amount of data which must be transmitted over a low bandwidth channel in order to reconstruct an acceptable video image of a presented document. For example, documents which are nominally bilevel (for example, black characters on white paper) can be digitized by assigning a one bit binary value, representing black or white, to each picture element or pel in the image captured by the camera. The binary data can then be compressed using known one-dimensional or two-dimensional run length encoding techniques.
Another technique for achieving greater reductions in delay time and communication costs is to reduce the image sampling rate. Instead of sampling the image at 40 picture elements or pels per inch, the sampling rate may be reduced to 20 or even 10 pels per inch. The amount of data which must be encoded and transmitted is directly related to the sampling rate.
However, video images captured at reduced sampling rates can become distorted. One type of distortion that can occur is along a vertical edge of an image area; for example, along a vertical edge of the stem in a typed or printed "T". What may appear, at a distance, as a perfectly straight edge will turn out to have irregularities on closer inspection. Depending upon the relative orientation of the sampling pel positions and the vertical edge, simply reducing the image pels to one bit binary values may cause the irregularities to be accentuated, thus degrading the recreated video image.
To avoid image degradation along vertical edges, vertical filtering may be performed to reduce pel-to-pel variations in a vertical direction. According to one known technique, the digital value of substantially every pel in the image is modified as a function of the captured digital values for both of its vertical neighbors; that is, the pels above and below the pel being modified or filtered. If the vertical neighbor pels have higher digital values than the pel being filtered, the digital value of the pel is increased. Conversely, if the digital values for the vertical neighbor pels are lower, the digital value of the pel being filtered is reduced.
While the results achieved by this technique are satisfactory, the problem is that a special purpose parallel processor must be provided. Without such a processor, the time required for image processing is significantly increased since substantially every pel is being filtered. The parallel processor adds undesirable cost to a teleconferencing system.